Add a means of extracting active operations from SkPicture

Add a means of extracting active operations from SkPicture

For the "pull forward" task I will be comparing the two cases:
  analyze the whole skp and use the BBH information
  analyze only the active portion of the skp

In the first case we need a way to get the BBH information out of the picture in order to extract the relevant portions of the whole-skp analysis. This adds caching of the active ops so that work isn't duplicated between when the optimization path queries for that information and when the usual draw path queries for it.

R=reed@google.com, bsalomon@google.com

Author: robertphillips@google.com

Review URL: https://codereview.chromium.org/195793010

git-svn-id: http://skia.googlecode.com/svn/trunk@13836 2bbb7eff-a529-9590-31e7-b0007b416f81